1. Field of the Invention
This invention relates to a meter driving device, and more particularly to a device for driving a cross coil meter which indicates, upon energization on a pair of crossed exciting coils, angular directions corresponding to input values.
2. Description of the Related Art
Heretofore, a cross coil meter for indicating angular directions corresponding to input values has been well known. The indication is made by applying, to a rotatably supported magnet, a torque originated from magnetic field which is generated by supplying electric currents corresponding to the input value to a pair of crossed exciting coils. Such cross coil meter is for use in, for example, speed meter, tachometer, fuel meter or oil pressure indicator of automobiles and the like.
FIGS. 5 and 6 of the accompanying drawings show schematic views of such a cross coil meter. A pair of exciting coils Ls and Lc which are arranged orthogonally to each other generate a magnetic field in a desired angular direction upon supplied electric currents corresponding to the input value, like of the speed. A rotatably supported permanent magnet M then receives a torque originated from the magnetic field generated at the two exciting coils Ls, Lc. As a result, a pointer fixed on the permanent magnet M goes rotating for an angle corresponding to the input value as shown in FIG. 6, thereby indicating a predetermined position of an indication plate on which a physical quantity (=speed in case of FIG. 6) to be measured is scaled.
A device for driving such a cross coil meter used as speed meter and fuel meter of automobiles etc., is generally composed: to input pulse signals, the frequency of which fluctuates depending on the input value e.g. of speed; also to input basic clock signals generated by a clock generating means; and then to count one cycle of the pulse signal using the pulses of the basic clock signals thereby obtaining the frequency of the input value.
Electric currents corresponding to the calculated frequency are modulated by PWM (pulse width modulation) and supplied to the crossed exciting coils Ls, Lc via a driver. For supplying currents corresponding to the frequency of the input pulse signal, pointer rotating angles for a several input values S are predviously determined, and these values (S, .theta.) are previously stored in a E.sup.2 PROM being a ROM capable of electrically erasing and writing data. For instance, as shown in FIG. 7, four pointer rotation angles .theta.1, .theta.2, .theta.3 and .theta.4 corresponding to input values S1, S2, S3 and S4 are stored as parameters in E.sup.2 PROM. To obtain each rotating angle .theta. for indicating the position corresponding to the input value S, the parameters are read out of the E.sup.2 PROM and the following calculation is executed: EQU .theta.=.theta.n+(.theta.n+1-.theta.n)/(Sn+1-Sn)*(S-Sn) . . . (1)
where n=1, 2, 3, 4.
Thereafter a predetermined process is performed in accordance with the obtained .theta. in order to supply currents to the coils Ls, Lc.
In this manner, it is possible to drive a variety of meters by a single driving device, by storing rotating angles .theta. for the predetermined input values S in the E.sup.2 PROM and calculating rotating angles corresponding to the input values to be indicated, and by using the stored data. Namely, due to the difference in indication system of meters depending on the types of automobiles etc., it had been required to prepare different driving systems for driving the meters. In this regard, by adopting E.sup.2 PROM, the driving system can deal with such differences only by suitably rewriting the data (S, .theta.) being stored therein.
But such a conventional meter driving device must use the formula (1) to obtain pointer rotating angle .theta. corresponding to the input value S, resulting in complicated calculation program. That is, it has been necessary to incorporate the program for division calculation (.theta.n+1-.theta.n)/(Sn+1-Sn) as shown in the formula (1) into an IC of the driving device, and this brings increased number of ROMs to complicate process steps.
In addition, on driving a cross coil meter, the exciting coils are energized based on the cycle of an input pulse signal from a speed sensor or a revolution sensor detected by counting the number of pulses of a basic clock signal having a predetermined basic frequency (e.g. 2 MHz). However, since the maximum frequency i.e. minimum period of an input signal to be detected differs depending on the types of automobiles, there has been a disadvantage that the number of bits in the counter for counting the number of the basic clock signal pulses extensively increases.
Now, it is assumed that the maximum frequencies of the inputted pulse signals for the period detection mutually differ as 250 Hz, 500 Hz, 1 KHz and 2 Kz depending on the types of automobiles incorporating the meter. On counting the cycle of the input pulse signal by means of basic clock signals for restricting the cycle detection error accompanying with the quantization error arising at that counting to a predetermined value (normally below 1/1000), the number of basic clock pulses becomes necessary to be more than one thousand. Specifically, when the maximum input frequency is expressed as fmax, the basic clock frequency must be set at over 1000 fmax, so when fmax equals to 2 KHz, over 2 MHz.
When the frequency of the basic clock signal is 2 MHz, then, the counter for counting the number of pulses of this clock signal must judge whether the input frequency is 0 Hz or 2 Hz at the indication resolution of 1/1000. If the judging line is established at 1 Hz, the pulse number to be counted will be 2,000,000 requiring a counter of 21 bits.
However, at counting the cycle of the input pulse signal with basic clock signals having a predetermined frequency of 2 MHz, a counter of 21 bits suffices for an input signal having a maximum frequency of 2 KHz, but for an input signal having a maximum frequency of 1 KHz, a counter of 22 bits will be required because of the cycle increasing twice that of 2 KHz signal. Moreover, for an input signal having a maximum frequency of 500 Hz, a quadruple i.e. 23 bits-counter, and further for an input signal having a maximum frequency of 250 Hz, an eight-fold i.e. 24 bits-counter must be provided respectively.
Of course, the accuracy enhances proportionally to the increase of the number of bits. Nevertheless, since the angular direction to be indicated has significant relationship to the fixed resolution property of the meter, it is neither practical nor effective to enhance the accuracy recklessly.
Furthermore, in the conventional meter driving device, it has been disadvantageous that the indication corresponding to the input value could not be performed accurately in particular at the time of turning ON or OFF of the power source in the driver, since the meter has been drived by directly supplying PWM-modulated currents to the driver.
The cause of this disadvantage; when the automobile ignition switch is in OFF state, the driver is generally also in OFF state and receives no voltage for avoiding its unnecessary large power consumption upon supplied currents due to its analog system. When the ignition switch and the power source are turned ON, the voltage of the driver builds-up to be ready to function. However, in this initial state, the processing data is unsettled yet. So the energizing of the exciting coils by the driver based on such indefinite value would cause inaccurate indication of the meter not corresponding to the input value.
Likewise, at the OFF state of the power source in the driver, the driver can not act correctly because of the lowering of the voltage below the operating voltage, thereby causing abnormal meter operation.